The present invention relates generally to improvements in devices for the collection of solar thermal energy and more particularly pertains to new and improved solar heated ponds.
One method that has been used for the efficient collection of low temperature solar thermal energy has been the use of flat plate collectors. Such collectors are efficient because their transparent glass covers freely pass the sun's visible radiation into their interior while being opaque to the infrared radiation from the heated interior. In this manner, the collectors efficiently trap the heat in their interior.
The use of flat plate collectors for production of low temperature solar energy on a large acreage basis is prohibitively expensive, however, because of the need for a large amount of glass pane which is a high cost material. The mounting and maintenance of the glass is an expense which cannot be economically justified. The problem becomes one of devising a collector functionally similar to the glass covered flat plate collectors but which is suitable for acreage size installations. Substituting a maintenance free relatively inexpensive cover for the fragile glass cover is one approach.
Another approach is to use a solar heated pond in which a shallow pool of liquid, usually a special salt solution, is exposed to the incoming sunshine. If the liquid is kept clean and clear, incoming sunshine can penetrate the body of liquid and warm the bottom layers of the pond. The warm bottom layers then radiate infrared energy which is trapped by the liquid in the layers above it, acting in the same fashion that the glass does in the flat plate collectors.
The problems which have held back solar ponds from coming into competitive use are (1) the need to keep the liquid free from deposited dust, which destroys its transparency to visible light; (2) the need to assure that the hot fluid at the bottom of the pond does not circulate by convection and bring the heat close to the exposed upper surface of the pond for reradiation; and (3) the need to minimize evaporation from the exposed surface of the pond. There is also considerable difficulty involved in extracting heat from solar ponds since the removal to a heat exchanger, for example, and subsequent replacement of the hot bottom liquid of the pond induces convection.